Casus belli

A casus belli, sometimes abbreviated CB and meaning "cause for war" in Latin, represents a justification for war that is recognized as legitimate by other characters. A ruler must have a valid casus belli in order to be able to declare war upon another ruler and can only gain from a war what the casus belli specifies.

War target

The war target is the map region which is targeted by a casus belli. If the casus belli does not target a map region then the defender's realm capital becomes the war target.

The attacking side gains more war score by occupying holdings locates in the war target region. After a grace period the defender will passively gain war score by holding all counties in the war target region.

CB list

Each casus belli will cost  prestige and/or  piety. If the attacker can't pay the cost, it can pay with a level of fame or devotion instead. Declaring war against a character's head of faith costs  500 piety if the head of faith is spiritual and  100 piety if they are temporal. Declaring war against a theocracy of the same faith additionally costs  100 piety, stacking with the cost for attacking the head of faith.

Casus belli costs can be removed via one of the game rules.

Casus belli	Requirements	Cost	Enforce Demands effect	White Peace effect	Surrender effect
Independence War	Target is the liege	500 prestige if not tribal  250 prestige if tribal	Attacker becomes independent	Attacker remains a vassal	Attacker remains a vassal Defender imprisons the attacker Defender gains title revocation reason
Claims	Claim on the target  Divine Right innovation to press several of the attacker's claims in a single war  Rightful Ownership innovation to press several of another character's claims in a single war	Prestige per title: Rank Own claim Other's claim County 100 250 Duchy 200 500 Kingdom 400 1000 Empire 800 3000	Claimant gains the targeted titles Attacker gets a  weak hook on claimant	Defender keeps their titles	Defender keeps their titles Claimant loses the claims used
Subjugate?	One of the following: Tribal government, Distinguished level of fame, and the target has tribal government Eastern religion and Exalted among Men level of fame, and the target follows an Eastern religion Temporal head of faith and Devoted Servant level of devotion, and the target shares the attacker's faith	1000 prestige if not tribal  500 prestige if tribal	Attacker becomes the defender's liege	Defender remains independent	Defender remains independent Attacker's factions gain discontent
Raid for Captives	Faith has the Human Sacrifice tenet	80 if faith has the Armed Pilgrimages tenet 100 if faith doesn't have the Armed Pilgrimages tenet	Attacker imprisons 3–5 of the defender's courtiers	Attacker gets no prisoners	Defender imprisons 3–5 of the attacker's courtiers if their faith also has the Human Sacrifice tenet
Excommunication War	Same faith Target is independent Target has the Excommunicated trait	None	Defender suffers succession	Defender keeps their titles	Defender keeps their titles Defender loses the Excommunicated trait Attacker's head of faith loses opinion of them
Ducal Conquest of County	Conquest lifestyle perk Attacker owns a county in the same uncreated duchy	350 prestige if defender's rank is count  750 prestige if defender's rank is higher than count	Attacker gains the targeted county	Defender keeps their titles	Defender keeps their titles Attacker's factions gain discontent
Forced Vassalization	Forced Vassalage lifestyle perk Target is an independent neighbouring realm Target's realm is not too large Target is not a holy order or head of faith	125 prestige per county	Attacker becomes the defender's liege	Defender stays independent	Defender stays independent Attacker's factions gain discontent
Laudably Conquer County	Request Claim on Ireland decision Target is in the Hibernia region Attacker and defender don't have the same liege	150 prestige if feudal and different faith  75 prestige if tribal and different faith  100 piety if feudal and same faith  50 piety if tribal and same faith	Attacker gains the defender's titles	Defender keeps their titles	Defender keeps their titles Attacker's factions gain discontent
Mongol Invasion of Kingdom	Mongol Empire title Established level of fame Tribal government Realms are adjacent		Attacker takes all titles from Hostile or Evil faiths Attacker vassalizes all rulers of other faiths	Defender keeps their titles	Defender keeps their titles Attacker's factions gain discontent
Restore Imperial Province	Roman Empire title Duchy was part of the Roman Empire Religion is Christian or Greco-Roman	None	Attacker gains the targeted Duchy	Defender keeps their titles	Defender keeps their titles Attacker pays reparations

De jure CBs

Where a title belongs to another title de jure (a county to a duchy, duchy to a Kingdom etc.), the holder of the higher title has a de jure CB towards the character who holds the lower title if they're not already a vassal. All de jure CBs cost  100 prestige per county in the war target title and have a war contribution share of  300 prestige.

• If the war ends by enforcing demands, the attacker will gain the target title. If they're a vassal, their liege will gain a  weak hook on them.
• If the war ends by white peace, the defender keeps the target title.
• If the war ends by surrender, the defender keeps the target title and the attacker pays  gold to the defender.

War target rank	Required innovation
County	Casus Belli
Duchy	Chronicle Writing
All titles	Rightful Ownership

Conquest CBs

The following CBs are only available to rulers who have clan or tribal government or whose faith has the Warmonger or Pursuit of Power tenets.

• If the war ends by enforcing demands, the attacker will gain the target title and fame equal to the war contribution share.
• If the war ends by white peace, the defender keeps the target title and the attacker loses  75 prestige.
• If the war ends by surrender, the defender keeps the target title, while the attacker pays the defender  gold and gains +25 faction discontent.

War target rank	Required level of fame	Same faith cost	Different faith cost	War contribution share	Once per lifetime
County	Established	50 if tribal 100 if clan	75 if tribal 150 if clan	75
Duchy	Illustrious	250 if tribal 500 if clan	375 if tribal 750 if clan	150
Kingdom	Exalted Among Men	2000	2000	350

Holy war CBs

The following CBs will cause a holy war instead of a regular one. This requires the Faith to be organized and to not have a tenet that forbids holy wars, such as Pacifism or Dharmic Pacifism. In any case, it can only target a ruler whose faith is considered Hostile or Evil.

• If the war ends by enforcing demands, the attacker will gain all titles held by rulers of Hostile or Evil faith and vassalize all other rulers in the war target region. The defender gains twice the  fervor the attacker paid.
• If the war ends by white peace, the defender keeps all their titles and the attacker loses  50 piety.
• If the war ends by surrender, the defender keeps all their titles, while the attacker pays  gold and gains +25 faction discontent and  −15 vassal opinion for 5 years.

War target rank	Required level of devotion	Fervor cost	Piety cost (without Armed Pilgrimages tenet)	Piety cost (with Armed Pilgrimages tenet)	War contribution share	Once per lifetime
County	Dutiful	1	100	80	100
Duchy	Faithful	2	200	160	200
Kingdom	Paragon of Virtue	5	750	600	500

Migrate to Pannonia

Migrate to Pannonia is a unique casus belli with special mechanics that can only be used once per game by characters of Táltosist faith who make the Prepare to Cross the Carpathians decision against characters in the Pannonian Basin region. If the Hungarian Migration game rule is set to Immediate, the game will start with the casus belli being used by Grand Prince Álmos Ügyekfi of the Mogyër Confederation against King Boris of Bulgaria.

If the war ends in white peace, the defender keeps his or her titles. If it ends in surrender the attacker's factions will also gain discontent. If the war ends by enforcing demands however it will have the following effects:

• Attacker gains all titles held by the defender in the Pannonian Basin region.
• Attacker gains the Kingdom of Hungary title, which replaces Kingdom of Pannonia.
• Any other kingdom title held by the attacker is destroyed.
• All held titles that are not within the de jure Kingdom of Hungary are given to new randomly generated characters of their culture and religion.
• All vassals that are not within the de jure Kingdom of Hungary become independent.
• All controlled counties convert to Mogyër culture and Táltosist faith.
• All previously controlled counties convert to Pecheneg culture.

When war is declared with this casus belli, the following army will be spawned, which will persist for as long as the attacking character lives or until destroyed or disbanded:

Years	Levies	Horse Archers
867–889	3000	2000
890–904	5000	3000
905+	6000	4000

Varangian Adventure

Varangian Adventure is a unique casus belli with special mechanics that can only be used during the Tribal Era on counties and duchies owned by characters from a different culture group. It costs  1000 prestige. To be used it requires the Varangian Adventurers innovation, which is only available to North Germanic cultures and requires the Northern Lords DLC. It can be used multiple times but each use will cost than the previous one. It cannot be used by kings or emperors.

If the war ends in white peace, the defender keeps his or her titles. If it ends in surrender, the attacker must also pay  gold and  prestige to the defender. If the war ends by enforcing demands however it will have the following effects:

• All titles held by the attacker are destroyed or given to other characters.
• All vassals are given independence.
• Attacker gains the target title and all titles that are de jure part of it.
• Attacker gains the  Adventurer trait.
• Attacker gains  200 gold.
• If the target title was a county and its de jure duchy does not exist or is part of the defender's realm, the duchy is squarted upon.

When war is declared using this casus belli for the first time, it will spawn for the duration of the war the following army based on the number of owned counties of the same faith and culture in the region of Mainland Scandinavia:

Counties	Varangian Veterans	Bondi	Vigmen	Huscarls
1	100	0	0	0
2	300	0	0	0
3	300	200	0	0
4	300	400	0	0
5	300	400	200	0
6	300	400	400	0
7	300	400	600	0
8	300	400	800	0
9	300	400	800	200
10	300	400	800	400

The army will always include  100 levies. If the character's dynasty has the first Adventure legacy, it will gain an additional  100 levies, 400 Varangian Veterans, 100 Bondi and 400 Vigmen.

AI rulers will only use this casus belli if all their held titles are located in the Mainland Scandinavia region. If the Scandinavian Adventurers game rule is not set to Apocalyptic then they also require the  Ambitious trait, as well as at least two of the following traits:  Adventurer,  Arrogant,  Brave,  Greedy.

Notes

Mechanics